Hyperspectral imaging is essentially a three-dimensional imaging method, whereby the two spatial dimensions of an image are augmented by a third dimension in which the wavelength spectrum of each pixel is encoded. The result is a hyperspectral “data cube” of information, which can be highly useful in a wide range of application, including mining and geology (e.g. looking for ores and petroleum deposits), agriculture, and military surveillance.
A hyperspectral camera collects data as a set of images, where each image represents a wavelength range or band of the electromagnetic spectrum, where the detected wavelengths need not be limited to the visible spectrum. Depending on the wavelength range of each image, the method may be referred to as “ultraspectral” (for very fine spectral resolution), “hyperspectral” (for intermediate spectral resolution), or “multispectral” for fewer and/or broader spectral bands that span a wide range of wavelengths. All such related methods are referred to generically herein as “hyperspectral” unless otherwise required by the context.
Because each image produced by a hyperspectral camera includes only the light from a single wavelength band, in contrast with a “panchromatic” image that combines light over a wide range of wavelengths, a hyperspectral image will generally have a lower resolution than a panchromatic image obtained under similar conditions.
There are two basic types of camera for obtaining hyperspectral data. A “staring array” hyperspectral camera repeatedly images an entire scene through a tunable filter while advancing the wavelength of the filter so as to obtain the images for the different wavelength bands. One advantage of this approach is that the detected wavelength bands need not be contiguous, but can be selected according to the wavelength bands of interest for a particular application. Nevertheless, while staring array hyperspectral cameras are useful for some applications, the limited tuning rate of the optical filter can limit the data throughput.
The other basic type of hyperspectral camera is a “push-broom” hyperspectral camera. In contrast to a staring array camera, which images an entire scene at once and then steps or sweeps through a series of wavelength bands to obtain the data cube, a push-broom hyperspectral camera obtains only one line of an image at a time, but simultaneously obtains the full spectral information for each point on the line. The imaging line is then stepped or swept through the image to provide the complete data cube. Push-broom imaging is especially useful for monitoring industrial processes, where items are typically moving past the camera at a constant speed. Similarly, push-broom imaging is well suited to airborne applications, where the camera is steadily moved across a scene. In such applications, the push-broom approach of measuring the full wavelength information of a line at the same instant ensures that all the wavelength information is truly measured from the same portion of the scene, even though the scene is moving during the measurement.
Push-broom cameras obtain wavelength spectrum information by focusing an image of a scene onto a line-selecting slit, and then either reflecting the light that passes through the slit from a reflective grating, or by passing the light through a transmission grating or through a diffraction slit. The latter case is an especially simple and effective approach, because a single slit can serve as both the line-selecting slit and the diffraction slit.
The push-broom design avoids the delays associated with filter tuning in a staring array camera. However, a push-broom camera cannot select specific wavelength bands, but can only provide spectral information over a continuous range of wavelengths.
FIG. 1A is a perspective view illustrating the basic components of a single-slit push-broom hyperspectral camera of the prior art. A lens 100 focuses an image 102 onto image panel 104 that is penetrated by a thin and narrow diffraction slit 106. Typically, the diffraction slit 106 is wide enough to span most of the image 102. For example, typical dimensions might be a 25 mm diameter circular image 102 focused onto a 20 mm wide slit 106 that is 20 microns high.
Light 108 that passes through the slit 108 falls on a hyperspectral detector array 110 mounted on a detector panel 112. The data collected by the detector array 110 represents spatial information along one axis 114, and wavelength information along the other axis 116. Repetition of this measurement during movement 118 of the scene past the lens, or movement 118 of the camera past the scene, provides information in the other spatial dimension, and allows collection of the complete data cube.
FIG. 1B is a detailed front view of the image 102 focused on the slit 108 of FIG. 1A. Note that the height of the slit has been exaggerated to make it perceptible in the drawing. It is evident that most of the image 118 focused by the lens 100 does not pass through the slit 106 and is not used.
It is sometimes desirable to compensate for the shortcoming of a hyperspectral camera by simultaneously gathering data from the scene using other types of detectors as “companion” sensors. For example, the lower resolution of a hyperspectral camera can be compensated by simultaneously detecting light from the scene using one or more panchromatic sensors. Also, light can be detected from additional spectral bands of interest, for example light resulting from “Light Detecting and Ranging” (LIDAR), by using separate sensors that are either intrinsically sensitive to the wavelength bands of interest or include appropriate filters.
However, including companion sensors can require that the light from the lens pass through a splitter and be shared between the hyperspectral sensor and the companion sensors, thereby reducing the light available to the hyperspectral camera, and reducing the quality of the hyperspectral result. Another possibility is to provide light to the companion sensors using a separate optical system. However, this approach consumes significantly more space than the hyperspectral camera by itself. Also, it can be difficult to register the data from the companion sensors with the hyperspectral image, due to inevitable misalignments between the two optical systems.
What is needed, therefore, is an apparatus for detecting push-broom hyperspectral data cube images that includes companion sensors but does not reduce the amount of light reaching the hyperspectral sensor and does not require separate companion sensor optics.